1. Field of the Invention
The present invention relates to a battery voltage equalizer circuit and a method for using the same, and more particularly, to a circuit that is capable of equalizing a battery voltage of each of a plurality of batteries in a serial connection.
2. Description of Related Art
With the development of portable electronic products, demands for rechargeable batteries consequently have been increasing. Rechargeable batteries include conventional nickel-cadmium batteries, developed nickel-metal hydride batteries, lithium-ion batteries, and the latest developed lithium-polymer (Li-Polymer) batteries. The voltages supplied from different types of rechargeable batteries are not the same and the required operating voltage for each of portable electronic products is not the same as well. Therefore, the battery manufacturer connects different numbers of the batteries in a serial manner to form a battery module for providing the required operating voltage for each of the portable electronic products.
Battery modules are needed to be recharged for next usage by chargers while the battery modules are out of power. However, electric power storage of batteries may not be at the same level due to many reasons, such as various manufacturing and environmental conditions. For Example, a 7.4 V Li-Polymer battery module would have two 3.7 V Li-Polymer batteries in a series connection. Initially, while the two Li-Polymer batteries are placed in the market stream the electric power storage thereof may respectively stay at 80% and 70%. Because the rechargeable lithium batteries would be damaged while they are overcharged, the charger for the rechargeable lithium batteries stops charging the batteries as long as any of the rechargeable lithium batteries is fully-charged. Thus, the electric power storage of the two Li-Polymer batteries could be respectively at 100% (maximum of battery capacity) and 90% after charged. When the electrical power storage of either one of the two Li-Polymer batteries decreases to 0% of the maximum battery capacity, the battery module is shut down while the electric power storage of another Li-Polymer batteries remaining at 10% of the maximum battery capacity.
According to the aforementioned example, the equivalent power storage of the battery module is determined by the battery with the lower electric power storage of the battery module. Furthermore, the batteries themselves would be subject to self-discharging with different self-discharging rates. Therefore, the electric power storage of the batteries in the same battery module may be further imbalanced, thereby reducing the actual usable electric energy of the battery module and shortening the life time.
Please refer to FIG. 1, in which a schematic diagram of the digital battery equalization controller in accordance with the product ISL9208 datasheet for Intersil is demonstrated. A digital battery equalization controller 10 comprises a microprocessor-controlled battery equalizer 5 and transistor switches S1˜S7. The transistor switches S1˜S7 are connected with batteries BAT1˜BAT7 through resistors R1˜R7 in a parallel connection. The voltages of the battery BAT1˜BAT7 are converted to digital signals via analog to digital converters (A/D Converter). Herein, the microprocessor controlled battery equalizer 5 determines the battery with higher battery voltage among the batteries BAT1˜BAT7 in response to the digital signals indicative of the battery voltages based on a built-in algorithms and then turns the transistor switch associated with the battery of higher battery voltage on. Thus, charging currents for each of the batteries could be adjusted with respect to the voltage of the batteries, thereby achieving the goal of equalizing the charging of the batteries.
Since the digital battery equalization controller 10 uses the A/D converter to convert the battery voltages to the digital signals in the above example, the overall chip size of the digital battery equalization controller 10 may become larger and thus production cost could be relatively higher. Meanwhile, the digital battery equalization controller 10 is subject to the limitation inherent in the initial circuit design that is only suitable for the battery module having 5 to 7 batteries in total.
Furthermore, the circuit may discharge the batteries with higher voltages until all of the battery voltages are equal, unless some circuit error occurs, e.g., over-temperature, over current, or short circuit. Li-Polymer batteries are considered that are charged or discharged with insignificant battery memory effect regardless of the voltage level thereof. However, discharging capability of the Li-Polymer batteries under different battery voltages would still be negatively affected as shown in FIG. 2, which is a schematic diagram illustrating a relationship between the battery voltage versus the discharge capability when the fully-charged Li-Polymer battery module has been used. Therefore, voltage equalization arbitrarily performed could cause a recession in discharging performance for a Li-Polymer battery module and an electronic product which applied the Li-Polymer battery module could even encounter a crash or corruption due to instantaneous power shortage.